Chi-square test in mendelian ratios is an important for getting answer to the question in any genetic experiment is how we can decide if our data fits any of the Mendelian ratios that we discussed earlier. A statistical test that can test out ratios is the Chi-Square or Goodness of Fit test.

The chi-square formula is as follows:-

- Degrees of freedom (d.f.) = n-1 where n is the number of classes
- Let us test the following hypothetical data to determine if it fits a 9:3:3:1 ratio.

Number of classes (n) = 4

Degrees of freedom (d.f.) = n – 1 = 4 – 1 = 3

Chi-square value = 0.47

- By statistical convention, we use the 0.05 probability level as our critical value.
- If the calculated chi-square value is less than the table chi-square value at the applicable degree of freedom (in this case, 3), we accept the hypothesis.
- If the value is greater than the table value, we reject the hypothesis. In this case, the table Chi-square value at d.f. = 3 is 7.82. Therefore, the calculated chi-square value (0.47) is less than the table value, and hence, we accept the hypothesis that the data fits a 9:3:3:1 ratio.